Thank you for your question regarding static control and grounding, it is our pleasure to help.
When it comes to static control and materials, it is best to think of the story of the Goldilocks and the Three Bears. All materials fall into one of three categories: Too resistive, too conductive, and just right.
If a material is “too resistive”, it means that the static charge that forms on it will not be able to dissipate. Think of the tape dispenser on your desk, when you pull off a piece of tape, there is something like 3,000 volts of static charge formed on one side of the tape. It will stay there, because there is no conductive path for it to move off. On a larger scale, if you consider a large sheet of plastic, say 8-ft x 4-ft, even if you ground one corner, static charges will still stay on the opposite corners of the sheet, as the material is too resistive to allow the static charge to cross its surface to get to the grounding conductor. In fact, for most plastics, static charges will not move across their surfaces even a millimeter to get to ground. The surfaces of most plastics are almost always to resistive to allow any static charge movement, but are great at allowing those static charges to form.
If a material is “too conductive”, it means that the static charge can move easily and rapidly across a given material. Think of anything metallic. If your sensitive electronic gear is in the path of a rapidly moving static charge, it is susceptible to the destructive forces of that charge.
The trick to static control is to get materials that are “just right”. You don’t want the charge to stay on a surface, waiting for an unintended discharge, and you don’t want it to move so quickly as to cause damage. Take the sheet of plastic from above, and let’s say it was manufactured with a slightly conductive material that lowered the resistance of the plastic, so that it would slowly conduct static charges. In other words, the static charge would slowly move across its surface to the grounded corner and out to earth. A slowly moving static charge will not damage electronics.
To answer your question, even if your urethane product is grounded, unless the urethane itself is manufactured with conductive materials in the correct proportions, it can indeed hold a significant static charge. Like the piece of tape, even a small area on the surface of your product can hold significant static charges. It is not unusual to see plastic objects with charges in certain zones that are in excess of 20,000 volts!
There are a number of static measurement devices available on the market, however proper training is critical. Particularly when dealing with the simple hand-held devices. The “Human Body Model” comes into play when using that device and can easily give you erroneous readings if it is not properly understood. The best way to measure your material is in a lab which should be able to give you a specific resistance. You may wish to contact your Injection Molding company and ask them about static control additives for the urethane. These additives, while adding some very minimal costs, are really quite effective at eliminating the static control problems, particularly given that your device is grounded. A good source of information is the Electrostatic Discharge Association and their ANSI/ESD S20.20 Program which provides excellent guidelines for testing and confirming static control procedures.
We hope you find this information useful, and please accept our apologies for the delay in answering your question. One of our engineers has an extensive back ground in static control, but was out on the road until today and we really wanted him to review this email prior to sending it. We hope you understand. Feel free to call us at 310-318-7151 if you have any further questions.
The Engineering Team at E&S Grounding Solutions
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